1. Field of the Invention
The present invention relates to electrical components, such as capacitors, and to methods for their manufacture wherein a porous conducting body is impregnated with a ceramic material, where the ceramic material is not an oxidized form of the body.
2. The Related Art
There are numerous ways to construct electrical components such as ceramic capacitors. In a conventional multilayer capacitor-making process, green (unfired) ceramic sheets having thicknesses of several hundred micrometers or less are prepared by tape casting, for example. Noble metal electrodes are screen printed onto the green ceramic by means of a conductive ink. The metals used must have relatively high melting points and be nonreactive at elevated temperatures, because the dielectric in the capacitor must be sintered at elevated temperatures.
In multilayer capacitors, sheets of green ceramic and electrodes are stacked on top of each other, with the electrodes staggered and partially overlapping each other, such that every other electrode extends to one end of the ceramic. Green ceramic sheets may be applied to the top and bottom of the stack in order to protect the outer electrodes and enhance durability of the device.
The stacks are then cut and fired at temperatures of up to 1350.degree. C. or higher, depending on the dielectric used, in order to properly sinter the ceramic dielectric. The ends of the device are then coated or terminated with a conductive metal or mixture, to connect the alternate electrodes within the device.
In an effort to use lower cost materials which do not require the same high temperature properties, metal impregnation techniques have been used whereby fugitive electrodes are screen printed on the green ceramic by means of an ink comprising a carbon powder with a binder and solvent.
After stacking layers of green ceramic and fugitive electrodes, the device is first heated to a low temperature of approximately 350.degree. C. in order to burn out the fugitive electrode ink present. The device is then fired so the ceramic dielectric can properly sinter at temperatures up to 1350.degree. C. This leaves gaps into which true metal electrodes are infiltrated after the dielectric is sintered.
In the case of metal impregnated electrodes, the end termination connects the alternative spaces within the device. Then the device is dipped into a molten metal (for example, lead) bath and the pressure controlled so as to fill molten metal into the layers between the dielectric. The end termination must be performed before the lead impregnation step, in order to prevent the leak of molten lead from the electrode layers when removed from the molten bath.
The advantage of the metal impregnation technique is that lower cost metals can be used as electrodes, because the electrodes are not put in place until after the high temperature sintering of the ceramic dielectric has taken place.
A disadvantage of both methods described above is the numerous processing steps required, including tape preparation, ink mixture, and numerous firing cycles.
If the electrodes are initially printed within the green ceramic, then expensive electrode materials such as platinum and palladium must be used to survive the high sintering temperatures required to sinter the dielectric without oxidizing or otherwise corroding the electrode.
For the metal impregnation method, problems with leaking of the electrode metal and joining leads to the termination ends of the capacitor sometimes occur.
A different type of capacitor is the tantalum type, which is formed by oxidizing a porous tantalum plug to form a tantalum oxide, typically tantalum pentoxide, as a dielectric film coating the tantalum. First, tantalum powder is formed into a shape and sintered into a porous tantalum plug. An electrode wire may be placed into the powder at the time the plug is sintered in order to insure good contact between the electrode and the tantalum.
Second, the tantalum is oxidized so that a dielectric layer of tantalum oxide, presumably tantalum pentoxide, forms on the surface of the tantalum plug. This may be accomplished using an electrolytic process. Then the plug is soaked in a solution of Mn(NO.sub.3).sub.2, which may help to heal any imperfections in the tantalum oxide layer. The plug is then heated and dried, and these steps may be repeated several times to insure an adequate oxide layer coats the surface of tantalum plug.
Third, the dielectric oxide coated plug is soaked in a conductive mixture one or more times to coat the oxide layer. Finally, the plug is dipped into silver paint and dried one or more times. The silver paint acts as a solderable conductor.
One such tantalum type capacitor is described in U.S. Pat. No. 4,160,284 issued to Deffeyes et al., which describes a method for forming capacitors by oxidizing a porous plug of metal to form a dielectric surface and impregnating the plug with a conductive metal paste.
Limitations of the tantalum type capacitor include the complex processing steps and the limited choice of dielectric, since the dielectric is limited to that of oxides of the metal plug. As a result, tantalum type capacitors generally have a fixed capacitance and cannot be used for a wide variety of applications.
It would be desirable to construct a capacitor requiring fewer processing steps than the conventional methods outlined above. For uses where multilayer capacitors are currently used, it would be desirable to omit the dielectric tape formation step. It would also be desirable to eliminate the electrode ink formation and application to the tape. In addition, it would be desirable to lower the sintering temperature requirements, thus allowing the replacement of precious metals with less expensive metals.
Regarding tantalum type capacitors, it would be desirable to be able to use a variety of dielectric materials with the same porous metal plug, in order to fabricate capacitors of varying capacitance, depending on the application desired. It is to these types of objectives that the present invention is directed.